diff --git a/setup.cfg b/setup.cfg
index b8fff52..6dd9111 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -53,3 +53,6 @@ files =
     tconnectsync
 follow_imports = silent
 ignore_missing_imports = True
+# Third-party deps such as requests ship no type stubs; treat them as untyped
+# instead of failing (older mypy does not silence this via ignore_missing_imports).
+disable_error_code = import-untyped
diff --git a/tconnectsync/api/tandemsource.py b/tconnectsync/api/tandemsource.py
index a530715..a02aeb2 100644
--- a/tconnectsync/api/tandemsource.py
+++ b/tconnectsync/api/tandemsource.py
@@ -221,6 +221,8 @@ class TandemSourceApi:
         # US default would send EU accounts to the US endpoints (#152).
         if not region:
             region = secret.TCONNECT_REGION
+        if not region:
+            raise ValueError("No region configured. Set TCONNECT_REGION to 'US' or 'EU'.")
         self.region = region.upper()
         if self.region not in ['US', 'EU']:
             raise ValueError(f"Invalid region '{region}'. Must be 'US' or 'EU'.")